1. Field of the Invention
The present invention is related to Integrated Circuits and, more particularly, to integrated circuits with Off Chip Drivers having switching noise compensation.
2. Background Description
Switching noise on power supply lines in Integrated Circuit (IC) packages is a well known problem. Typically, switching noise is the result of a high rate of change of current (di/dt) in the supply line. The noise is the voltage that develops across parasitic inductance (L) in the supply line. Thus, switching noise is, simply, L(di/dt).
Off Chip Drivers (OCDS) are the primary source of switching noise in state of the art IC packages, especially for complementary insulated gate technology (referred to in the art as CMOS) ICs. Each time an OCD switches its output's state, the supply current to the OCD changes from almost no current to a relatively large current and, then, back to almost no current in a few nanoseconds (ns).
This OCD switching current is additive. So, several OCDs switching simultaneously increase switching current (and di/dt) proportionately. Consequently, regardless of how small L is made, eventually, as the number of simultaneously switching OCDs increases, switching noise can become unacceptable.
One source of OCD switching current is a condition that occurs as the OCD is about to switch from one state to another, i.e., from a high to low or vice versa. Typically, an overlap period occurs when the OCD is, effectively, driving in both directions, i.e., a pull up device is pulling the output high and a pull down device is pulling the output low. This overlap and the resulting overlap current, occurs because one device is turning on before the other device has completely turned off. As a result, several techniques have been used to reduce or eliminate the overlap current.
In one such prior art technique, a logic input to the OCD gradually turns off the on-device before turning on the off-device. This prior art technique requires very sensitive OCD timing because, the logic signal must begin (e.g., rise) in anticipation of the change in data and last (remain high) until the on device is turned off, without substantially delaying turning on the off device and, thereby, unduly slowing the output transition. This prior art noise reduction technique also slows forcing tri-state OCDs into their high impedance state.
Thus, there is a need for reducing overlap current in OCDs.